The aliquot part of the distillate used should not contain more than
one-tenth of a gram of furfurol. The precipitate formed in this way is
collected on an asbestos felt, dried at 103° and weighed. The weight
obtained divided by 1.974 gives the corresponding amount of furfurol.
There is some difficulty experienced in loosening the precipitate from
the sides of the tubes in which the heating takes place, but this
defect can be overcome by heating in covered beakers in an autoclave.
=157. Precipitation with Phloroglucin.=—Instead of using pyrogalol for
the precipitating reagent phloroglucin may be employed. The method of
procedure proposed by Councler for this purpose is given below.[131]
The furfurol is prepared by distillation in the usual way. The volume
of the distillate obtained is completed to half a liter with twelve per
cent hydrochloric acid, and an aliquot portion, varying in volume with
the percentage of furfurol is withdrawn for precipitation. This portion
is placed in a glass-stoppered flask with about twice the quantity of
finely powdered phloroglucin necessary to combine with the furfurol
present. The contents of the flask are well shaken and allowed to stand
fifteen hours. The precipitate is collected on an asbestos filter,
washed free of chlorin, dried at the temperature of boiling water and
weighed.
The theoretical quantity of precipitate corresponding to one part of
furfurol, _viz._, 2.22 parts, is never obtained since the precipitate
is not wholly insoluble in water. The actual proportions between
the precipitate and the original furfurol vary with the amount of
precipitate obtained.
When the weight of the precipitate is 200 milligrams and over, 2.12
parts correspond to one part of furfurol. When the weight of the
precipitate varies from fifty to 100 milligrams, the ratio is as 2.05:1
and when only about twenty-five milligrams of precipitate are obtained
the ratio is as 1.98:1.
The quantity of pentose bodies corresponding to the furfurol is
calculated from the factors given by Tollens in a preceding paragraph.
The reaction which takes place with furfurol and phloroglucin is simply
a condensation of the reagents with the separation of water. It is very
nearly represented by the following formula:
2C₅H₄O₂ + C₆H₆O₃ = C₁₆H₁₂O₆ + H₂O.
Furfurol. Phloroglucin Condensation Water.
product.
It has been shown by Welbel and Zeisel,[132] that in the presence of
twelve per cent of hydrochloric acid phloroglucin itself is condensed
into dark insoluble compounds. When three molecules of furfurol and two
molecules of phloroglucin are present, the bodies are both condensed
and separated by continued action. When from one and a quarter to three
parts of phloroglucin by weight are used to one part of furfurol, the
weight of the precipitate obtained under constant conditions may serve
sufficiently well for the calculation of the furfurol. The precipitates
contain chlorin, which they give up even in the cold, to water. For
these reasons the analytical data obtained by the method of Councler,
given above, are apt to be misleading. It is probable also that
similar conditions may to a certain extent prevail in the separation
of furfurol with phenylhydrazin, and further investigation in this
direction is desirable. For the present the very best method that can
be recommended for the estimation of pentoses and pentosans is the
conversion thereof into furfurol and the separation of the compound
with phenylhydrazin acetate.
=158. Estimation of Sugars by Fermentation.=—When a solution of
a hexose sugar is subjected to the action of certain ferments a
decomposition of the molecule takes place with the production of
carbon dioxid and various alcohols and organic acids. Under the action
of the ferment of yeast _Saccharomyces cerevisiae_ the sugar yields
theoretically only carbon dioxid and ethyl alcohol, as represented by
the equation: